Duplex image forming apparatus

Abstract

An image forming apparatus reverses a sheet, on one side of which an image has been formed in an image forming section, transports the sheet back to the image forming section, and forms an image on the other side of the sheet, the apparatus has an inlet feed path for transporting a sheet, on one side of which an image has been formed; a switchback path for withdrawing the sheet transported from the inlet feed path; a plurality of reverse feed paths branching from the switchback path at plural positions, each of the reverse feed paths transporting a sheet while reversing the sheet; and a re-feed path for transporting the reversed sheet from any of the reverse feed paths back to the image forming section.

What is claimed is:  
     
       1. An image forming apparatus adapted to reverse a sheet on one side of which an image has been formed in an image forming section, transport the sheet back to said image forming section, and form an image on a second side of the sheet, the apparatus comprising: 
       an inlet feed path for transporting a sheet, on one side of which an image has been formed;  
       a switchback path for withdrawing the sheet transported from said inlet feed path;  
       a plurality of reverse feed paths each branching from said switchback path at one of plural positions, each of said plurality of reverse feed paths transporting a sheet while reversing the sheet; and  
       a re-feed path for transporting the reversed sheet from any of said plurality of reverse feed paths back to said image forming section.  
     
     
       2. An image forming apparatus according to  claim 1 , wherein after the sheet is withdrawn into said switchback path, the sheet is transported backward, selectively transferred to one of said plurality of reverse feed paths, and transported through the selected reverse feed path, whereby the sheet is transported to said re-feed path after being reversed. 
     
     
       3. An image forming apparatus according to  claim 2 , further comprising a plurality of reversible sheet feed means for transporting a sheet to said switchback path, wherein the sheet is transported into said switchback path with forward rotation of said sheet feed means, and thereafter the sheet having been transported into said switchback path is transported to one of said plurality of reverse feed paths with backward rotation of said sheet feed means. 
     
     
       4. An image forming apparatus according to  claim 2 , wherein while a preceding sheet is transported in a reverse direction in each of said plurality of reverse feed paths, a succeeding sheet transported from said inlet feed path is transported in a direction in which the succeeding sheet is withdrawn into said switchback path, such that the preceding sheet and the succeeding sheet pass each other at a branch position in which said reverse feed path is branched from said switchback path. 
     
     
       5. An image forming apparatus according to  claim 4 , further comprising a plurality of roller pairs provided in said switchback path and each pair contactable with each other and moveable apart from each other, wherein when a sheet transported into said switchback path is transported to said reverse feed path, said pair of rollers are moved apart from each other, thereby enabling the preceding sheet and the succeeding sheet to pass each other at said branch position. 
     
     
       6. An image forming apparatus according to  claim 1 , wherein said switchback path is disposed linearly and substantially parallel to a sheet feed direction as the sheet leaves said image forming section, and said re-feed path is disposed linearly and substantially parallel to said switchback path, said inlet feed path transporting a sheet to said switchback path after reversing the sheet and said plurality of reverse feed paths transporting respective sheets to said re-feed path after reversing the sheets. 
     
     
       7. An image forming apparatus according to  claim 6 , wherein said switchback path and said re-feed path are disposed substantially in a horizontal direction. 
     
     
       8. An image forming apparatus according to  claim 1 , further comprising control means for controlling sheet transport such that sheets successively transported to said switchback path at a predetermined interval are distributed to said plurality of reverse feed paths in a predetermined order and are transported to said re-feed path in a predetermined feed order. 
     
     
       9. An image forming apparatus according to  claim 8 , wherein said control means includes means for controlling and temporarily stopping a sheet transported in said switchback path and a sheet transported in said re-feed path. 
     
     
       10. An image forming apparatus according to  claim 9 , wherein, under control of said control means, succeeding sheets are stopped temporarily in said plurality of reverse feed paths while a sheet is stopped temporarily in said re-feed path, and after the sheet in said re-feed path has been transported out of said re-feed path, the temporarily stopped sheets in said plurality of reverse feed paths are transported to said re-feed path in the predetermined feed order. 
     
     
       11. An image forming apparatus according to  claim 9 , wherein, under control of said control means, sheets stopped in said plurality of reverse feed paths are transported to said re-feed path in the predetermined feed order at an interval shorter than a predetermined interval between the sheets transported to said switchback path. 
     
     
       12. An image forming apparatus according to  claim 1 , further comprising control means for changing an interval between sheets, which are successively transported to said switchback path, depending on a sheet length in a sheet feed direction. 
     
     
       13. An image forming apparatus according to  claim 1 , wherein said inlet feed path is provided in plural, and each inlet feed path merges with said switchback path. 
     
     
       14. An image forming apparatus according to  claim 13 , wherein after transporting a sheet to said switchback path from one of said plurality of inlet feed paths and withdrawing the sheet into said switchback path, the sheet is transported in a reverse direction, selectively transferred to one of said plurality of reverse feed paths, and transported through the selected reverse feed path, whereby the sheet is transported to said re-feed path after being reversed. 
     
     
       15. An image forming apparatus according to  claim 14 , further comprising a plurality of reversible sheet feed means for transporting a sheet to said switchback path from one of said plurality of inlet feed paths, wherein the sheet is transported into said switchback path with forward rotation of said sheet feed means, and thereafter the sheet having been transported into said switchback path is transported to one of said plurality of reverse feed paths with backward rotation of said sheet feed means. 
     
     
       16. An image forming apparatus according to  claim 13 , wherein said switchback path is disposed linearly and substantially parallel to a sheet feed direction in said image forming section and said re-feed path is disposed linearly and substantially parallel to said switchback path, wherein said plurality of inlet feed paths transport respective sheets to said switchback path after reversing the sheets and said plurality of reverse feed paths transport respective sheets to said re-feed path after reversing the sheets. 
     
     
       17. An image forming apparatus according to  claim 16 , wherein said switchback path and said re-feed path are disposed substantially in a horizontal direction. 
     
     
       18. An image forming apparatus according to  claim 13 , further comprising control means for controlling transport of the sheets to successively transported the sheets from said plurality of inlet feed paths, and successively transport the sheets from said plurality of reverse feed paths to said re-feed path in the same order as that in which the sheets have been transported from said plurality of inlet feed paths to said switchback path. 
     
     
       19. An image forming apparatus according to  claim 18 , wherein said control means further controls the sheets successively transported to said plurality of inlet feed paths to stand by in said plurality of inlet feed paths, said switchback path, said plurality of reverse feed paths and said re-feed path in predetermined order, and the standing-by sheets are then transported to said re-feed path at predetermined timing in predetermined order. 
     
     
       20. An image forming apparatus according to  claim 19 , further comprising a sheet feed path provided between a sheet container containing sheets which are not yet subjected to image formation and said image forming section, and merging with said re-feed path, wherein said control means further controls the sheet standing by in said re-feed path to be transported to said sheet feed path after sheets in number capable of standing by in said plurality of inlet feed paths, said switchback path, said plurality of reverse feed paths and said re-feed path have been transported to the side downstream of a merging portion of said sheet feed path and said re-feed path. 
     
     
       21. An image forming apparatus according to  claim 20 , wherein said control means further controls the sheets so that after transporting the sheet standing by in said re-feed path to said sheet feed path, a sheet contained in said sheet container is transported to said sheet feed path before a next standing sheet is transported to said sheet feed path, whereby the sheet, which is not yet subjected to image formation and the standing-by sheet are alternately transported. 
     
     
       22. An image forming apparatus according to  claim 13 , further comprising a first sheet ejection path for ejecting a sheet having an image formed on one side immediately after image formation, and a second sheet ejection path for ejecting a sheet having an image formed on one side after reversing the sheet, wherein a sheet is reversed through a predetermined one of said plurality of inlet feed paths and said switchback path, and the reversed sheet is ejected through said second sheet ejection path. 
     
     
       23. An image forming apparatus according to  claim 22 , further comprising control means for controlling sheet feed interval such that a space corresponding to at least one sheet is left before or after the sheet that is transported through said second sheet ejection path after being reversed. 
     
     
       24. An image forming apparatus according to  claim 22 , further comprising a sheet feed path provided between a sheet container containing sheets which are not yet subjected to image formation and said image forming section, and merging with said re-feed path, and control means for, after sheets each having an image formed on one side have been ejected after being reversed, transporting sheets to said sheet feed path in the same number as that of the sheets which have been ejected after being reversed. 
     
     
       25. An image forming apparatus according to  claim 13 , further comprising control means for changing an interval between the transported sheets depending on a sheet length in a sheet feed direction. 
     
     
       26. An image forming apparatus adapted to reverse a sheet on one side of which an image has been formed in an image forming section, transport the sheet back to said image forming section, and form an image on a second side of the sheet, the apparatus comprising: 
       a reverse inlet feed path for transporting a sheet, on one side of which an image has been formed in said image forming section, while reversing the sheet;  
       a linear switchback path for withdrawing the sheet transported from said reverse inlet feed path;  
       a plurality of reverse feed paths each branching from said switchback path at one of plural positions, each of said plurality of reverse feed paths transporting a sheet while reversing the sheet;  
       a linear re-feed path provided substantially parallel to said switchback path and transporting the reversed sheet from any of said plurality of reverse feed paths back to said image forming section; and  
       flappers provided at respective positions where said plurality of reverse feed paths are branched from said switchback path, said flappers positioned to selectively guide the sheets from said switchback path to said plurality of reverse feed paths.  
     
     
       27. An image forming apparatus according to  claim 26 , further comprising a plurality of reverse inlet feed paths for transporting respective sheets, on one side of each of which an image has been formed in said image forming section, to said switchback path after reversing the sheet; and flappers for selectively guiding the sheets, on one side of each of which an image has been formed, to said plurality of reverse inlet feed paths. 
     
     
       28. An image forming apparatus according to  claim 27 , wherein said switchback path and said re-feed path are disposed substantially in a horizontal direction. 
     
     
       29. An image forming apparatus according to  claim 26 , wherein said switchback path and said re-feed path are disposed substantially in a horizontal direction.